Isotopic composition of Pb in tree rings of Norway spruce (Picea abies) from the Fensfjorden area, western Norway

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Abstract

The annual growth of tree rings makes the trees as
possible archive of the past atmospheric
pollution. However, several aspects addressed in
literature such as post-depositional mobility of
Pb, intra-ring heterogeneity and possible
detoxification mechanisms can make the
interpretation on past atmospheric pollution,
difficult. Annual rings of two Norway spruce
(Picea Abies) trees 43 and 42 years old located
10.1 km north-northwest and 17.8 km east-southeast
of Mongstad refinery, respectively, were analysed
by laser ablation inductively coupled plasma mass
spectrometer (LA-ICP-MS) for lead isotopic
composition, and 63Cu/12C, 66Zn/12C and 44Ca/24Mg
elemental ratios. These two trees, recorded
206Pb/207Pb ratios between 1.13 and 1.32, and 1.12
and 1.29, respectively, but showed no systematic
trend towards higher or lower ratios from the
innermost to the outermost tree ring. For all
ratios, only the 208Pb/12C (for tree #1) and
44Ca/24Mg (for tree #1 and #2) ratios showed
pronounced trends. The 208Pb/12C ratio of tree #1
increased from 1968 and peaked in the year 1979,
followed by a steady decrease toward the present
(2010), and showed significant correlation with
the lead content and the pH of precipitation in
the area. The 44Ca/24Mg of tree #1 and #2
demonstrated showed a systematically decrease from
1968-2010 and showed significant correlation with
the pH of the precipitation. Comparison of the
lead isotopic composition of the tree rings with
that recorded by mosses in the same area during
the time period 1975-2000 showed significant
differences. Most of the tree rings plotted have
higher 206Pb/207Pb and 208Pb/207Pb ratios relative
to the Norwegian peat bogs (1691-1991), and also
show some overlap with the relatively higher
recorded ratios of a Greenlandic ice core (7313 BC
to 1523 AD). The tree rings have also higher Pb
isotopic ratios compared to the European aerosols.
Comparisons with the 206Pb/207Pb ratios recorded
by the Scottish peat bogs and mosses covering the
same time period as the trees did not reveal any
similarity. The onset of production at Mongstad
refinery in 1975 and the subsequent expansion in
the refinery production and occasional accidents
in the factory did not have any significant impact
on lead isotopic composition of the tree rings,
nor on the 208Pb/12C, 63Cu/12C, 66Zn/12C or the
44Ca/24Mg ratios. Pollution associated with the
Mongstad refinery may be minor compared to the
long range pollution by atmospheric aerosols. In
order to differentiate between pollution sources
and natural sources, local soil and bedrock
measurements and a chemical and isotopic analysis
of Pb in oil that is being refined at Mongstad,
should be carried out.